NO. 2297. BRACHIOPOD GENUS PLATYSTROPHIA—McEWAN. 



397 



It seems to the writer that the species of Platystrophia represent 

 the "ever-sporting varieties" of De Vries, and the different muta- 

 tions are marked by variation in the phcation pattern. The propa- 

 gation of these mutations is most prominent in the Low Fold and 

 Ponderosa subgroups and rare in the High Fold subgroup. 



This development is so pronounced in PlatystropJiia elegantula, 

 new species, that it has become of specific value. Platystrophia 

 narrowensis also sliows a prolific development of these individuals. 



ORTHOGENESIS. 



In order to simplify the discussion of the life history it is thought 

 advisable to construct first a diagram as a guide to the discussion 

 which will follow. The purpose of the diagram is to illustrate the 

 common characters of the various groups and subgroups into which 

 the genus can be subdivided and the common order of appearance 

 of these characters. The line represents the ontogeny and it is 

 subdivided into the ontogenetic stages which are indicated by let- 

 ters. The early onto stages are so condensed as to make it impos- 

 sible to measure with accuracy their relative duration, but an effort 

 is made to indicate where a developmental stage is noticeably shorter 

 in one group than in another. 



<o fPrimitiye ancestor. 



a g < P. uniplicata A B 



no 



t3 



Specialized members A B 



Subgroup A A B 



Specialized members A B 



Subgroup B • A B 



Specialized members .'. A B 



Subgroup C A B 



Specialized members A B 



C D 



C D 



C D 



C D 



C D 



Subgroup D A B 



C D 



Specialized members A B | C D 



Low Fold subgroup A 



E El 



E E» 



El 



El 



E2 



•CO 



Ponderosa subgroup A 



High Fold subgroup A 



MORPHIO STAGES. 



A. Smooth protegulum stage. 



B. Ribbed stage. 



C. True ventral sinus and brachial fold developed. 



D. Plications added on lateral slopes. 



E. Plication in sinus bifurcates and plication intercalated in a median position on fold. 



El. Plication intercalated in a median position in the sinus and median plication on the fold bifurcates. 

 E'. Plication implanted on each slope of sinus and primary phcation of fold bifurcates. 



F. Increase in size. 



G. Elevation of fold. 



H. Excessive increase in size. 



I. Specialized and individual characters. 



